In Rutherford spectroscopy, modeling this effect with a potential that decays as (read slowly) “one over r, times e to the minus r” prevents the cross-section from diverging. Adding this effect to a helium atom’s Hamiltonian reduces the variational parameter from 2 to “27 over 16.” This effect is estimated by: adding multiples of (20[1])0.3, (0[2])0.35, 0.85, or 1 (20[1])at quantum numbers n (20[1])and n minus 1, then subtracting the sum (0[1])from atomic number to give (*) “Z-effective,” when (0[1])using (10[1]0[2])Slater’s rules. (10[2])This (10[1])effect and (10[1])orbital penetration explain why ionization (10[1])energy decreases down (10[3])a group. (10[1])The lanthanide contraction occurs because 4f electrons do (10[1])not participate in (10[2]0[1])this effect. For 10 points, name this effect in which core electrons block (10[3])the (10[1])nucleus’s tug (10[1])on (10[4])valence (10[6])electrons. ■END■